A passive optical network (PON) is a point-to-multipoint network architecture in which unpowered optical splitters are used to enable a single optical fibre to serve multiple premises. A PON typically includes an Optical Line Terminal (OLT) at the service provider's central office connected to a number (typically 32-128) of Optical Network Terminals (ONTs), each of which provides an interface to customer equipment.
In operation, downstream signals are broadcast from the OLT to the ONTs on a shared fibre network. Various techniques, such as encryption, can be used to ensure that each ONT can only receive signals that are addressed to it. Upstream signals are transmitted from each ONT to the OLT, using a multiple access protocol, such as time division multiple access (TDMA), to prevent “collisions”.
A Wavelength Division Multiplexing PON, or WDM-PON, is a type of passive optical network in which multiple optical wavelengths are used to increase the upstream and/or downstream bandwidth available to end users. FIG. 1 is a block diagram illustrating a typical WDM-PON system. As may be seen in FIG. 1, the OLT 4 comprises a plurality of transceivers 6, each of which includes a light source 8 and a detector 10 for sending and receiving optical signals on respective wavelength channels, and an optical combiner/splitter 12 for combining light from/to the light source 8 and detector 10 onto a single optical fibre 14. The light source 8 may be a conventional laser diode such as, for example, a distributed feed-back (DFB) laser, for transmitting data on the desired wavelength using either direct laser modulation, or an external modulator (not shown) as desired. The detector 10 may, for example, be a PIN diode for detecting optical signal received through the network. An optical mux/demux 16 (such as, for example, an Arrayed Waveguide Grating—AWG—, or a Thin-Film Filter—TFF) is used to couple light between each transceiver 6 and an optical fibre trunk 18, which may include one or more passive optical power splitters (not shown).
A passive remote node 20 serving one or more customer sites includes an optical mux/demux 22 (which may, for example, also be an AWG or TFF) for demultiplexing each wavelength channels (λ1 . . . λn) from the optical trunk fibre 18. Each wavelength is then routed to an appropriate PON 24 comprising one or more Optical Network Terminals (ONTs) 26 at respective customer premises. Typically, each ONT 26 includes a light source 28, detector 30 and combiner/splitter 32, all of which are typically configured and operate in a manner mirroring that of the corresponding transceiver 6 in the OLT 4.
The WDM-PON illustrated in FIG. 1 is known, for example, from “Low Cost WDM PON With Colorless Bidirectional Transceivers”, Shin, DJ et al, Journal of Lightwave Technology, Vol. 24, No. 1, January 2006. With this arrangement, each PON 24 is served by a predetermined pair of wavelength channels, comprising an L-band channel for downlink signals transmitted from the ONT 4 to the respective PON 24, and a C-band channel for uplink signals transmitted from the respective PON 24 to the ONT 4. The MUX/DEMUX 16 in the ONT 4 directs couples the selected channels of each PON 24 to a respective one of the transceivers 6. Consequently, each transceiver 6 of the ONT is associated with one of the PONs 24, and controls uplink and downlink signalling between the ONT 4 and that PON 24. Each transceiver 6 and 26 in the OLT 4 and ONTs is rendered “colorless”, by using reflective light sources 8, 28, such as, reflective semi-conductor optical amplifiers; injection-locked Fabry-Perot lasers; reflective electro-absorptive modulators; and reflective Mach-Zehnder modulators. With this arrangement, each light source 8, 28 requires a “seed” light which is then modulated to produce the respective downlink/uplink optical signals. In the system of FIG. 1, the seed light for downlink signals is provided by an L-band broadband light source (BLS) 32 via an L-band optical circulator 34. Similarly, the seed light for uplink signals is provided by a C-band broadband light source (BLS) 36 via a C-band optical circulator 38. As is known in the art, the BLSs 32 and 36 may be broadband light emitting sources that generate a continuous spectrum, such as a Light Emitting Diode (LED), or may be a multi-frequency laser source which generates a plurality of narrow-band lights.
WDM-PONs suffer limitations in that the fibre trunk 18 and the BLSs 32, 36 constitute single points of failure of the entire network. A failure of any one of these components effectively disconnects all subscribers.
A typical method for implementing WDM-PON protection is to duplicate the WDM-PON system of FIG. 1, with both systems being connected to the same ONTs 26. A less expensive alternative, which provides trunk fibre protection, is illustrated in FIG. 2. As may be seen, this technique employs a switch 40 at each end of the trunk fibre, so that either one of two (or more) alternative fibre routes 42 can be switched in as needed. However, each of the switches 40 requires electrical power in order to operate. In the case of the switch 40alocated at the OLT 4, this does not pose a serious problem. However, the remote node 20 is typically passive, and so does not have an electrical power supply. Consequently, installation of an optical switch 40b at the remote node 20 also requires the provision of electrical power, which undesirably increases costs.
This problem can be overcome by replacing the switch 40b at the remote node 20 with a 3 dB combiner 43. However, this solution suffers a limitation in that the combiner introduces optical losses into the system. The theoretical loss is 3 dB, but in practice, losses of about 4 dB are highly likely. This represents a large loss in the WDM PON link budget. The link budget most affected by any additional loss is that of the C-band BLS 36 to the subscriber transceivers 26. A loss of 4 dB represents a fibre length of approximately 13 km, which in a network that is at maximum 20 km long is a considerable penalty.
Typically, a BLS 32,36 is protected using an optical switch 44, as shown in FIG. 2. This arrangement adds approximately 1 dB to the loss of the path. This further loss is added to the 3-4 dB losses caused by switching in the fibre trunk 18, and so compounds the performance penalty associated with the protection switching function.
Techniques for efficiently protecting fibre trunk and/or the Broadband Light Sources (BLSs) in a WDM-PON remain highly desirable.